Background/Purpose: Obesity is associated with an increased risk of osteoarthritis also in non-weight bearing joints and increased amounts of visceral fat are associated with lower bone density. This observation and the association of chronically elevated free fatty acid (FFA) serum levels with a number of inflammatory cardiovascular and metabolic diseases suggest that FFA may also play a role in inflammation-related bone loss. We therefore analyzed the effect of FFA on the central cells of bone metabolism, i.e. osteoblasts and osteoclasts, in the context of rheumatic diseases.

Methods: Primary osteoblasts (OB) were isolated from cancellous bone of OA and RA patients undergoing knee joint surgery. Osteoclasts (OC) were differentiated from peripheral blood mononuclear cells (PBMC). OB and OC were stimulated with the saturated FFA palmitic acid (PA) and the unsaturated FFA linoleic acid (LA). Immunoassays were used to quantify protein secretion. mRNA expression levels were quantified by real-time PCR. Mineralization activity was quantified using Alizarin Red S staining, differentiated OC were quantified by counting TRAP-positive multinuclear cells. Toll-like receptor (TLR) 4 and TLR2 were blocked by neutralizing antibodies.

Results: OB secreted increased amounts of the proinflammatory cytokine IL-6 (up to 9-fold ­) as well as the chemokines IL-8 (up to 221-fold ­), GRO-a (from below detection level to detectable levels) and MCP‑1 (up to 16-fold ­) upon stimulation with PA or LA. The degree of response was highly dependent on the patient. RANKL as well as OPG, important regulators of osteoclastogenesis and OC activity, remained unaffected by FFA on protein and mRNA level. OB activity appeared unaffected because alkaline phosphatase (ALP) and collagen type I mRNA expression as well as production of inorganic matrix was not altered. Differentiation markers of OB (e.g. osteocalcin) also remained unchanged after FFA stimulation. PA-induced IL-8 secretion by OB could be significantly reduced by TLR4 blockade (by 93%), while blocking TLR2 had no effect. Secretion of IL-8 by RA OC was increased by FFA, while MMP-9 was reduced. The number of TRAP positive multinuclear cells decreased (by around 50%). However, markers of osteoclast activity (CLCN7, CTSK, TCIRG) remained unchanged at the mRNA level.

Conclusion: Our results clearly suggest a pro-inflammatory effect of certain FFA on osteoblasts and osteoclasts. While this can indirectly contribute to bone loss, the reduced number of mature OC after FFA stimulation suggests an inhibitory effect on bone resorption. The effect of FFA on cells of bone metabolism therefore appears to be divergent. The effects on osteoblasts are at least in part mediated by TLR4, while TLR2 is not involved.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/bone-metabolism-in-rheumatic-diseases-may-be-affected-by-free-fatty-acids/